This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-280499, filed Sep. 30, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to an exposure mask suitable for various beam exposure schemes using a charged beam or X-rays and, more particularly, to an exposure mask in which positional distortion of a mask pattern due to internal stress is corrected, and a method of manufacturing the exposure mask.
In recent years, circuit devices represented by LSI devices are rapidly shrinking in feature size, and therefore photolithography currently used as a micropatterning technique on the commercial level will soon become unable to form a minimum line width of several ten nm or less because of its resolution capability limit. Demand has arisen for the development of a microfabrication technique that can take the place of the current photolithography. A candidate that can meet this requirement is lithography using short wavelength VUV (Vacuum Ultra Violet) light, X-rays, or an electron beam.
A 1X X-ray lithography exposure using X-rays irradiates a wafer 33 with X-rays 31 through an X-ray exposure mask 30 with a mask pattern to transfer the mask pattern onto an X-ray resist 32, as shown in FIG. 1. The X-ray exposure mask 30 has a structure having a 1- to 5-xcexcm thick membrane (X-ray transparent supporting film) 2 made of an element with light weight which transmits X-rays, such as silicon nitride, silicon carbide, silicon, or diamond, and an X-ray absorber pattern 4 formed on the membrane 2 to absorb X-rays, and is supported by a reinforcing frame 7 and Si substrate frame 1. The Si substrate frame 1 is used to minimize distortion in the X-ray absorber pattern 4 because the membrane 2 is very thin and mechanically weak.
To manufacture this X-ray exposure mask, first membrane films are formed on both surfaces of an Si substrate, and an etching stopper and anti-reflection film, X-ray absorber film, and pattern transfer etching hard mask film are formed on one surface. Next, the membrane film on the lower surface side of the Si substrate is removed by etching, the Si substrate at the opening portion is removed by back etching, and the Si substrate frame is reinforced by bonding a reinforcing frame. Then, a resist pattern is formed by electron beam exposure and development, the pattern transfer etching hard mask film is etched, and the resist is removed. Finally, the X-ray absorber film is etched, and the pattern transfer etching hard mask is removed to form an X-ray absorber pattern on the membrane film, thus completing to fabricate an X-ray exposure mask.
The in-plane pattern image placement accuracy of the conventional X-ray exposure mask obtained by the above method is evaluated by measuring displacements (in-plane distortion; IPD) of the formed pattern with respect to a desired pattern using an image placement measurement apparatus (LMS-IPRO available from Leica). As shown in FIG. 2, pattern displacements are observed. Referring to FIG. 2, the broken lines indicate the desired positions, and the solid lines indicate the actually measured positions.
Generated IPD values are 11 nm on average and 43 nm for 3"sgr". The maximum displacement amount is 49 nm. The displacements occurred almost in one direction. For example, the overlay accuracy required to realize a 150-nm rule device by X-ray lithography is 50 nm for ⅓ the design rule. If this includes 30 nm as a value allowable for the mask, the pattern accuracy measured above is insufficient.
Required image placement accuracy cannot be obtained probably because each of the resist film, pattern transfer hard mask film, absorber film, membrane film, and etching stopper film, which form the mask, has (1) internal stress, (2) film thickness distribution, and (3) internal stress distribution. The pattern position is displaced in proportion to the difference in the product of film thickness by stress (forces acting inside the film) in each region of films forming the X-ray exposure mask. As examples, FIGS. 3A and 3B show pattern distortions generated by tensile stress and compression stress of the X-ray absorber film, respectively.
When an X-ray absorber pattern is formed from the X-ray absorber film 4 having tensile stress at a desired (resist) pattern position by etching, the absorber pattern position is displaced outward from the center, as shown in FIG. 3A, because internal stress at the etched (removed) portion of the absorber film is released. On the other hand, when the X-ray absorber film has compression stress, the absorber pattern position after etching is displaced toward the central portion, as shown in FIG. 3B, relative to the (resist) pattern at the desired position. If the pattern density changes all over the pattern area, the pattern position is displaced from the desired position in accordance with the pattern density.
In not only the X-ray absorber film but all films forming the mask, if internal stress is present, a displacement from a desired position occurs. Since the pattern position is displaced in proportion to the product of film thickness by stress (forces acting inside the film), the pattern position is largely displaced from the desired position if the film thickness changes in the plane. Even when the film has internal stress distribution, IPD occurs.
In the conventional X-ray exposure mask manufacturing, mainly, the materials and processes are optimized to uniform the in-plane distribution of internal stress in each film of the mask, and for a film (except the membrane film) formed by etching, internal stress is reduced, thereby suppressing IPD. However, these methods are still poor in film reproducibility and have problems in obtaining a required image placement accuracy.
A method of improving image placement accuracy after mask formation has been proposed in which ions are implanted into a membrane film immediately under a pattern, and film stress is changed using the effect that the ions enter the crystal of the membrane film to increase the lattice spacing, thereby suppressing distortion attributed to an X-ray absorber pattern using the out-plane distortion of the membrane film (Jpn. Pat. Appln. KOKAI Publication No. 4-196211).
Another method of improving image placement accuracy after pattern formation has been proposed in which a membrane film is etched, or ions are implanted into the membrane film to increase/decrease the film thickness or film stress (Jpn. Pat. Appln. KOKAI Publication No. 7-94395). Still another method of improving the pattern accuracy of a mask has been proposed in which a stretchable film is deposited on the lower surface of a mask substrate around the X-ray exposure region, thereby reducing the concave mask distortion to planarize the film (Jpn. Pat. Appln. KOKAI Publication No. 5-90137).
In all of the prior-art techniques, any pattern distortion due to degradation in planarity can be reduced, though fabrication of a considerably uniform membrane film, non-warp frame, and X-ray absorber film controlled to low stress is required, resulting in technical difficulty. Requirements for in-plane uniformity such as film thickness and stress are also severe. When a membrane film is etched, the X-ray transparency largely changes to result in a variation in pattern size. In addition, since the membrane film serves as a supporter, its etching is undesirable from the viewpoint of stability.
An electron beam projection lithography mask using a reduction image projection technique, a SCALPEL (SCattering with Angular Limitation for Projection Electron Lithography) mask, or a stencil mask also has a structure in which an electron scattering pattern is formed on a membrane film, as in the X-ray exposure mask. Requirements for formation of a low-stress thin film and in-plane uniformity such as film thickness and stress are severe, and prior-art techniques have the same technical difficulty as in the X-ray exposure mask.
As described above, for the conventional X-ray exposure mask and electron beam exposure mask, to attain the image placement accuracy required for a micropattern with a minimum line width of several ten nm or less, each thin film of the mask is required to have values and uniformity that are very hard to achieve as physical properties including film stress, film stress in-plane uniformity, and film thickness. For this reason, it is difficult to realize a highly precise exposure mask which can use for further shrinking in feature size of a circuit device in the future.
It is an object of the present invention to provide an exposure mask which can accurately correct positional distortion due to internal stress of an X-ray absorber pattern or electron scattering pattern and has a high pattern accuracy to sufficiently use for shrinking in feature size in the future.
An exposure mask is formed by forming a mask layer on a transparent supporting layer (membrane film layer) having a relatively high exposure beam transparency. A mask layer of certain type has a transparent film having a relatively high exposure beam transparency and a mask pattern embedded in the transparent film and having a relatively low exposure beam transparency. The mask pattern sometimes has a positional error from a designed pattern. This is mainly because an in-plane stress distribution of the transparent film is nonuniform. This nonuniformity of the stress distribution is reduced by at least one stress adjusting portion to correct the positional error. Especially, when the stress adjusting portion is formed not on the transparent supporting film (membrane film) but on the transparent film, various advantages can be acquired.
As the stress adjusting portion, one of a cut portion obtained by partially cutting the transparent film, a transparent portion partially deposited on the transparent film, and a portion obtained by physically or chemically changing the transparent film is used.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.